There is known in the patent literature a great variety of systems and techniques for broadcast transmission. More particularly systems for controlling access to broadcast transmissions are described in the following publication:
Satellite and Cable TV Scrambling and Descrambling, by Brent Gale and Frank Baylin, published by Baylin/Gale Productions, Boulder, Colo., 1986.
Smart cards, plastic cards resembling credit cards with microprocessor inside of them, are known in the art. They are described in the book, Smart Cards, The New Bank Cards, by Jerome Svigals and published by Macmillan Publishing Company, N.Y., 1987.